A relatively recent development in the field of coatings and adhesives has been the development of a class of materials wich we shall refer to as polymeric organosilanes. These materials comprise organic polymer backbones having hydrolytically reactive silyl groups pendent from said backbones. These types of compounds can be conveniently produced by interpolymerizing ethylenically unsaturated organic monomers, e.g. ethyl acrylate, vinyl acetate and the like, with ethylenically unsaturated organosilane monomers having hydrolytically reactive groups bonded to the silicon, e.g. vinyltrimethoxysilane, .gamma.-methacryloxypropyl trimethoxysilane and the like. Examples of such interpolymerized organosilanes are found in U.S. Pat. Nos. 3,408,420; 3,306,800; 3,542,585; 3,962,471; 3,062,242; and 3,577,399.
The polymeric organosilanes described above are useful as coating materials, or, in some instances, to serve a function similar to that of a conventional coupling agent. In the latter case, the polymeric organosilanes are used in conjunction with conventional resinous coatings or adhesives to improve the compatibility and/or bonding between the resinous materials and inorganic oxide substrates or fillers. When the resinous material is a thermoplastic polymer containing essentially no chemically reactive functionality, the polymeric organosilanes may provide adhesion promoting benefits which are superior to those provided by conventional monomeric organosilane coupling agents. This is due to the fact that conventional organosilanes rely on the reactivity of their organo moiety with complementarily reactive groups in the resin matrix to provide bonding, whereas the polymeric organosilanes are believed to provide bonding to the resin matrix by non-reactive mechanisms such as through molecular entanglement with the molecules of the resin matrix and, to some extent, through Van Der Waals forces, thus, they do not require that the resin matrix contain reactive functional groups.
One of the major problems associated with the use of the polymeric organosilanes which has hindered their more widespread commercial use is their lack of shelf stability and potlife; that is, their tendency to undergo unacceptable viscosity increase or gelation prior to use. This lack of shelf stability is due to crosslinking reactions which take place at the pendent silyl groups of the polymeric organosilane molecules.
U.S. Pat. No. 4,043,953 discloses a means of improving the stability, i.e. potlife, of a coating composition based on polymeric organosilanes which are produced by copolymerizing an acrylatosilane, methacrylatosilane or vinyl silane with one or more vinyl-containing monomers, i.e., monomers containing the group CH.sub.2 .dbd.C&lt;. The coating compositions are stabilized against premature gelation by the addition thereto of from 0.5 to 15 percent based on the weight of the polymeric organosilane of a monomeric hydrolytically reactive silane of the formula X.sub.n Si(OR).sub.(4-n) wherein X is an organic radical having from 1 to 12 carbon atoms, R can be methyl, ethyl, 2-methoxymethyl, 2-ethoxyethyl, or an acyl group containing 5 or less carbon atoms, and n is 0, 1, or 2. It is believed that the addition of the monomeric silane stabilizes the polymeric silane coating composition by reacting with trace amounts of moisture and thereby preventing the moisture from hydrolyzing and crosslinking the polymeric organosilane.
While the method disclosed in U.S. Pat. No. 4,043,953 undoubtedly improves the stability of polymeric organosilanes, we have found that this method has certain limitations, especially when one desires to employ the polymeric organosilanes as adhesion promoter additives, rather than as coatings per se. For a number of reasons the viscosity stability requirements are somewhat more stringent when the polymeric organosilane is used as an adhesion promoter additive, rather than as a coating material. For example, when a copolymer of methyl methacrylate and gamma-methacryloxypropyltrimethoxysilane is employed as a coating composition, the gamma-methacryloxypropyltrimethoxysilane typically comprises about 5 to 10 weight percent of the copolymer. By contrast, when a copolymer of the same two monomers is to be employed as an adhesion promoter additive, the gamma-methacryloxypropyltrimethoxysilane typically comprises about 10 to 50 weight percent of the copolymer. Since only about one crosslink per 5,000 to 10,000 molecular weight units can cause the polymeric organosilane to gel, the relatively high silicon level which is desirable in polymeric organosilanes used as adhesion promoters increases the possibility of gelation by providing a relatively high number of sites for crosslinking.
When using a polymeric organosilane as a coating material, as disclosed in the aforementioned patent, U.S. Pat. No. 4,043,953, some viscosity increase is tolerable provided the composition is fluid enough to be applied to a substrate. By contrast, even relatively minor viscosity increases can inhibit the effectiveness of the polymeric organosilane when, for example, it is added to a thermoplastic adhesive to improve adhesion to an inorganic oxide substrate. To be effective, the adhesion promoter must migrate through the resin matrix to the substrate/adhesive interface and this becomes increasingly difficult as the viscosity of the adhesion promoter is elevated.
We have found that the addition of monomeric hydrolytically reactive silanes as stabilizers is not a very effective means of stabilizing the viscosity of a polymeric organosilane when the polymeric organosilane is exposed to moisture levels on the order of several thousand parts per million. Merely increasing the amount of monomeric hydrolytically reactive organosilane proportionately to the amount of water does not provide highly effective stabilization at these levels of water. Moreover, we have found that the amounts of monomeric hydrolytically reactive organosilane which would be required, on a stoichiometric basis, to scavenge water at levels of several thousand parts per million may impair the effectiveness of the polymeric organosilane as an adhesion promoter.
Since adhesion promoters are generally employed in small quantities, that is, on the order of a few weight percent of the resin to which they are added, they tend to be used rather slowly and may have to be stable over storage periods of several months. Moreover, they may be stored in containers which are frequently opened to remove small quantities and thus are highly susceptible to exposure to ambient moisture levels on the order of thousand of parts per million.
It is therefore, an object of this invention to provide a method of stabilizing a polymeric organosilane against viscosity increase or gelation in the presence of water without employing excessive amounts of monomeric hydrolytically reactive silanes that would impair the effectiveness of the polymeric organosilane as an adhesion promoter.
It is a further object of this invention to provide a highly stable polymeric organosilane composition, i.e., one which will not undergo gelation upon extended exposure to moisture levels on the order of several thousand parts per million.
Other objects and advantages of this invention will be apparent from the disclosures herein.